The invention relates generally to radiant energy imaging, and more particularly to a novel apparatus and technique for producing and directing high intensity X rays to an object of interest from a plurality of preselected points spaced from that object so as to obtain a cross-sectional image thereof. More specifically, the invention relates to a computed tomography (CT) X-ray scanner in which the X rays are produced using a source of high energy ions impinging on a target material.
The field of CT scanners has seen substantial improvements in recent years and has now advanced to a so called "fourth generation". Such a "fourth generation" device is disclosed in the copending patent application Ser. No. 726,556 of Stein, filed Sept. 22, 1976. That CT scanner includes an X-ray source (such as an X-ray tube containing a tungsten filament), which generates a fan beam and which is rotated in a circular arc about an axis along which a patient or other object to be examined is disposed. A stationary array of contiguous X-ray detectors is arranged in a circle outside the circumferential path of the X-ray source for converting incident X rays into corresponding detected electrical signals. The outputs from the X-ray detectors are logarithmically amplified, digitized and input to a computer for generating an array of pixel image values which are then displayed in two dimensions on a CRT screen as corresponding luminance values, representing a cross section of the patient or object.
A CT scanner which includes no moving parts is disclosed in U.S. Pat. No. 4,130,759 issued to Haimson. This device includes a fixed annular target from which an X-ray beam is selectively directed toward the patient. The X-ray beam is generated by a high power electron beam in a bell-shaped evacuated housing. The electron beam may be selectively electronically directed to any point on the target.
In each of the above-described CT scanners, the X rays are typically generated by a beam of electrons impinging on a target material such as tungsten. The electrons in the electron beam travel at a range of speeds and cause a broad Bremstralung spectrum of X rays to be emitted by the target when the electrons are decelerated by collisions with individual atoms in the target. The wide spectrum of X rays generated results in the well known "beam hardening" or "spectral hardening" problem. This problem results because the degree of X-ray absorption by the body is a function of the energy spectrum of the X rays (low energy X rays are preferentially absorbed), which function varies according to the particular body matter lying along the X-ray beam path.
As a result, the measured logarithmic attenuation of the X-ray beam is not a strictly linear function of the body thickness. Rather, the intensity of the X-ray beam received by an X-ray detector is a very complex function of the X-ray PG,4 energy spectrum and the quality and quantity of matter between the X-ray source and the X-ray detector. In order to eliminate or compensate for beam hardening, shields may be utilized to reduce the reception by the body of low energy photons and complex computer programs have been devised to compensate for the beam hardening which remains. However, these techniques have never been completely successful, primarily because of the nonlinear functional relationship between the output X-ray beam spectrum and the input X-ray spectrum for different materials in the X-ray path.